DEVICE LEVEL BUSSES AND DEVICE LEVEL NETWORKS IN WATER AND WASTEWATER FACILITIES

摘要

A Water or Wastewater treatment operation’s efficiency is rigorously constrained by its ability to effectively communicate with its associated field equipment and instrumentation. This communication manifests itself in both analog process data from field instrumentation and digital status data from field equipment and instrumentation. Control systems installed in treatment plants require communications to each device to retrieve the data for control, alarming, reporting, and presentation to the user. Communications have traditionally consisted of 4-20 mA analog data or contact closure discrete data to the control system modules. The common approach used to physically connect the various components of a traditional system utilized the “spider” method where one cable pair is wired to each device and piece of equipment.The current and future trend is to incorporate device level busses that use one cable to communicate to multiple devices. Device busses use high level communications to retrieve the device information for the control system controller, be it a programmable logic controller (PLC), distributed control unit (DCU), remote terminal unit (RTU), or other device. Today’s industrial busses are rapidly becoming the preferred method for communicating to field equipment, especially when considering installations in new and expansion construction projects. Using Device level busses for wiring installations provides the user with several inherent advantages over the traditional wiring methods. Some of these advantages include: 1. Reduction in wiring: The use of a common bus throughout the plant eliminates the need for dedicated wires between each device and the controller. 2. Reduced installation and startup time: A reduced probability of wiring errors with enhanced diagnostics from the device should reduce the time required to verify inputs/outputs (I/O). 3. Expanded information from devices. With bus level communications, more than one item of data can be retrieved from each device. 4. Lowered dedicated I/O requirements: Since each device is connected only to the bus cabling, the controller needs only the bus controller I/O, eliminating the need for an I/O point on a module for each point of data. 5. Expandability: Bus structures can be extended and additional devices added to the bus. The limit of each bus controller without the installation of additional control system modules must be considered however. 6. Diagnostics: With expanded information from devices, you will have the ability to send and receive diagnostic codes related to performance and troubleshooting information.